The long-term objective of the proposed research is to understand the role of sustained use of the eyes at near in the etiology of environmentally induced myopia. In this proposed research we are focusing on the role of individual differences of parameters of the oculomotor control system in the development of young-adult-onset myopia. The hypothesis of the proposed research is as follows: When the oculomotor system is subjected to prolonged use (as occurs in frequent and sustained reading at a close distance), an individual's resting accommodation, resting vergence, AC/A, CA/C, as well as plasticity of these elements contribute to the development of young-adult-onset myopia. The chief aims of the research are to determine: A) the contribution of tonic accommodation (Ta) and tonic vergence (Tv) to synergistic interactions of accommodation and vergence. We will test and refine a model of accommodative and vergence control that incorporates an accurate representation of the role of tonic innervation and accounts for adaptive variations of tonic input. We will be able to predict from this resulting model the demand on accommodation and vergence that occurs when a subject is required to perform a defined near task; B) the effect of individual differences in resting tonus of accommodation and vergence (Ta and Tv), and accommodation-vergence interactions (AC/A and CA/C) on the development of young-adult-onset myopia, and the correlation between the change of ocular optical components and the development of this myopia; C) how the plasticity of tonic accommodation and vergence are associated with the development of young-adult-onset myopia. Three experiments are proposed: In Experiment 1, vergence and accommodative responses will be measured before and after adaptation of tonic vergence while accommodative system is opened, and adaptation of tonic accommodation while vergence system is opened. This experiment is designed to test the validity of two alternative models that appear in the present literature. In Experiment 2, one-hundred university students will be followed over a three-year period. Refractive error, ocular optical components, and the parameters of accommodation and vergence, including their cross-links, will be monitored over this time span. These data from subjects who subsequently become myopic will be compared with those from subjects who remain emmetropic. The hypothesis is that specific parameters of the oculomotor control systems are different in young adults who become myopic as compared to those who do not. In Experiment 3, changes in tonic accommodation and vergence following defined visual tasks at near will be interpreted as oculomotor plasticity. Subjects' plasticity data will be compared to their records of refractive error. The results will be used to test the hypothesis that such oculomotor plasticity is a factor in the development of young-adult-onset myopia.